Very extensive discussion of the prior art appears in Bowker, mentioned above, and is earnestly commended to the attention of the reader. That discussion includes in particular a summary of previously known fingerprint techniques employing either frustrated total internal reflection (FTIR) or fiber-optic prisms. These topics will be only briefly summarized here.
FTIR Technology
In FTIR work, a so-called "critical angle" establishes key angular relationships between incident light and light-collection directions, for so-called "bright field" and "dark field" systems. As explained at length in Bowker, the critical angle is defined either by arcsin (1/n) or by arcsin (n'/n), both special cases of Snell's Law in which n represents the refractive index of the solid material of an optical block.
The first expression applies when air is at the surface of the block; the second, when some other medium is juxtaposed against the block--in which case n' represents the refractive index of that other medium. Conventional FTIR fingerprint systems direct incident light to a block surface, from within the solid material, at an angle which is intermediate between the critical angles for air and for typical biological materials such as skin or flesh, and water.
If no finger is present, or if a finger is present but a light ray strikes the surface at a groove of the fingerprint, then the light under consideration is all internally reflected into the block, from which it can be detected by a suitably positioned sensor. If instead a light ray strikes the surface at a ridge of a fingerprint, then part of this light passes through the surface and into the material of the finger where it is scattered diffusely--a certain fraction reemerging from the finger into the block, from which it, too, can be detected by a suitably positioned sensor.
The two suitable positions are quite different, leading to two different operating modes: a "bright field" mode in which the sensor is positioned to capture internally reflected light at air-filled fingerprint grooves (creating a bright field against which fingerprint ridges appear as relatively dark stripes), and a "dark field" mode in which the sensor is positioned to avoid capturing internally reflected light (in which case fingerprint ridges appear as relatively bright stripes of scattered light, against a dark field).
Dark-field systems, as explained at length in Bowker, are generally preferred for their higher contrast--ease of distinguishing ridges from grooves--and also from considering the ratio of signal to noise. Dark-field contrast is generally about unity, but bright-field contrast can be as low as about 1/7, with a proportionately lower signal-to-noise ratio. The only way to compensate in a bright-field system is to increase the exposure (that is, the light level or time, or their product) by about 7.sup.2 =forty-nine times.
As also noted in Bowker, earlier FTIR systems require a focal element such as a lens to image the FTIR data onto a detector array or scanning detector--but a lens has undesirable properties including focal distances ranging (for practical cases of interest) from very roughly 7 cm, with no magnification or reduction, to over 10 cm if magnification or reduction is needed.
A lens system is also susceptible to depth of field and distortion, particularly severe if the lens and object plane are not reasonably parallel and conaxial--as is typical in bright-field devices. Such systems have different magnifications, and severely divergent focal positions, too, at top and bottom of the fingerprint image, leading to complications in later interpretation of the acquired image.
Prior-art Fiber Prisms
Also discussed in Bowker are two prior patents proposing substitution of a fiber-optic prism for a clear prism, as a dark-field FTIR fingerprint collection block in a fingerprint reader: U.S. Pat. Nos. 4,785,171 and 4,932,776 of Dowling et al. Those patents appear to include several misunderstandings of the physical phenomena involved, leading to configurations that are very inefficient and marginally operative.
In his first patent, Dowling retains a collection lens spaced from the prism face, and injects light into his system at this same output face of his fiber-optic prism. This configuration is very vulnerable to scattering of the bright incident light by contamination at the common input/output face.
Dowling's '776 second patent acknowledges this problem, and teaches use of a fiber-optic taper, integral with the fiber prism, to match the print image to a relatively small CCD array. It also teaches--instead of the spaced-lens configuration with injection and detection at the same end of the fiber-optic element--attaching a CCD array directly to the end of the fiber taper remote from the finger, thus entirely eliminating the lens and associated optical gap.
The fiber core has refractive index 1.62 and the cladding 1.48, yielding against air a moderately high numerical aperture NA=0.66 and critical angle of about 38.degree.. This choice is conventional for obtaining good light-gathering power, although many skilled artisans in this field would prefer a considerably higher numerical aperture.
(For the majority of current applications involving fused-bundle faceplates or image conduits, glasses with numerical apertures of 1.0 and 0.66 are used. Fused-bundle materials are also available with a very few other numerical-aperture values such as 0.95, 0.85 and 0.35; however, 0.95 or 0.85 faceplate material is not always available, and 0.35 is typically run "infrequently due to lack of demand"--see for example "Fiber Optic Faceplate Data", Incom, Inc., Southbridge, Mass.).
Here Dowling sets out to apply the full capabilities of the tapered fiber prism to shorten the optical system, erect the image plane (supplying an image that is merely anamorphic but in uniform focus and free of major aberration), and eliminate or minimize effects of contamination and jarring. Unfortunately, however, Dowling's fiber prism is covered by a CCD at one end and a finger at the other, leaving no suitable entry point for illumination.
Dowling attacks this problem with three alternative tactics: transillumination of the fingertip, implanting lamps in the sensor end of the fiber prism, and directing light into the sides of the prism. It is shown in Bowker that all three suffer from major defects: very evident ones in the case of the first two tactics, and somewhat more subtle but still debilitating problems in the third.
As to the third tactic, illumination is specifically from the narrower sides of the taper--propagating toward the finger-contacting surface to be illuminated. In particular his illumination is directed into portions of the taper where fiber diameter is changing rapidly with respect to longitudinal position (i.e., the part of the taper that is actually tapered).
Analysis indicates that this Dowling system will at best work very poorly, and most likely not at all. In particular, the efficiency of light injection in this manner is extremely poor, and also would require a taper with no absorbing material outside the individual-fiber walls (usually designated "extra-mural absorbing" or "EMA" material)--thereby leading to severe fogging of the image.
If Dowling's apparatus has actually been made and operated, it must operate at the very bounds of usability--a power-hungry system working with small tail-end fragments of the input light that almost accidentally make their way to the fingerprint contact. It must have a low signal-to-noise ratio, due to massive diffusion of the backscattered light along the return path.
The Fiber-prism Systems Of Bowker
Bowker describes a solution using optical-fiber prism means that are crosslit. The prism means may be simply an optical-fiber prism, or may be a combination of such a prism with other elements such as an optical-fiber taper.
Light enters the prism means in a region where the fiber diameters are substantially constant with respect to longitudinal position, for lighting the fiber terminations where a fingertip is applied. Such illumination is enabled by use of a fiber prism in which the numerical aperture (NA) is radically low--by any of several different measures.
The NA preferably does not exceed one-half, and even more preferably does not exceed 0.42, and a preferred value that is available commercially is 0.35--at least in the region where the light crosses the fibers. The constraint on NA is also expressed in terms of other parameters.
In certain circumstances the prism means, at least in a region where the light crosses the fibers, have a numerical aperture NA that satisfies this maximum condition: EQU NA.ltoreq.2n.sub.avg (D/x.sub.F).sup.1/4, (Eq. 1)
where
n.sub.avg =average of core and cladding refractive indices in that region of the prism means; PA1 D=periodicity of the fiber structure in that same region; and PA1 x.sub.F =illumination-path distance across the prism means in that region, PA1 x.sub.M =illumination-path distance across the prism means to the prism midplane, in the same region. PA1 at the proper angle for FTIR operation (or reflection, as the case may be) at the fiber termination; and PA1 at an angle to the fiber which is not favorable to direct entry of the rays into a ducting mode. PA1 means for holding an electrical-energy storage device or for receiving electrical power from an external source, to power the apparatus; PA1 means for contacting a skin pattern to develop an electronic data array corresponding to an image of the skin pattern; PA1 means for generating in response a corresponding electronic data array for use in verification; PA1 means for performing a verification procedure; PA1 output means for indicating or effectuating, or both, a verification decision; PA1 means for formatting the data array in a compact form for use in storage, import or export; and PA1 means for converting the data array from said compact form to a different form for use by the verification-procedure performing means. PA1 means for holding an electrical-energy storage device or for receiving electrical power from an external source, to power the apparatus; PA1 means, including an imaging unit and a sensor array disposed to receive an image therefrom, for contacting a skin pattern to develop an electronic data array corresponding to an image of the skin pattern; PA1 a video controller for controlling the sensor array to develop said electronic data array; PA1 an analog-to-digital converter for digitizing the electronic data array; PA1 a digital signal processor for performing verification procedures based upon the electronic data array, and for developing a decision signal based upon the verification procedures; PA1 memory means for holding an authorized-user skin-pattern template, program firmware for the digital signal processor, and data used in the verification procedures; PA1 an output register for holding the decision signal; PA1 output means for transmitting a utilization-means switching signal, based on the decision signal, from the apparatus for effectuation of the decision signal; and PA1 a control, address, and data bus interconnecting the video controller, analog-to-digital converter, video processor, memory means, and output register. PA1 means for holding an electrical-energy storage device or for receiving electrical power from an external source, to power the apparatus; PA1 an optical bench disposed within or forming part of, or both, the case; PA1 optical-fiber prism means mounted to the optical-bench bosses for contacting a skin pattern to develop an image thereof; PA1 an objective lens mounted to the optical-bench ring for relaying the skin-pattern image to a sensor array; PA1 a sensor array mounted to the optical-bench pocket for receiving said image and in response developing an electronic data array corresponding to the image; PA1 a surface-mount electronics board holding a digital signal-processing chip for analyzing the data array to verify identity corresponding to such skin pattern; and PA1 verification-decision indicating or effectuating means, or both.
and the conventional notation (D/x.sub.F).sup.1/4 means the fourth root of the ratio D/x.sub.F.
In other cases, particularly having opposed light sources to illuminate the fiber terminations from both sides of a square-cut-off prism, preferably the prism-means numerical aperture is small enough--at least where the light crosses the fibers--that the projected light which crosses the entire prism means, from each side, has at least one hundredth of the respective initial intensity.
Alternatively, at least in a region where the light crosses the fibers, the numerical aperture satisfies a modified form of Eq. (4), EQU NA.ltoreq.2n.sub.avg (D/x.sub.M).sup.1/4, (Eq. 2)
where
Forms and variants of the teachings in Bowker include both bright- and dark-field systems, in many different prism configurations. One aspect of those teachings includes illumination by means of a partial reflector at an end of the prism means.
Placing Fiber-prism Images On A Sensor
In Bowker it is also taught that a sensor is advantageously mounted directly to the prism means--either directly to a primary prism that receives the fingertip whose pattern is to be analyzed, or directly to a fiber-optic taper that reduces the fingerprint image size for use with a much smaller sensor. These two forms of the sensor mounting taught in Bowker represent tradeoffs of the relatively high cost of sensors against the relatively high cost of tapers.
As pointed out in that document, the present price of even a relatively small detector if implemented as a conventional charge-coupled detector (CCD) array, is high enough to constitute the major cost element in apparatus according to the invention. A larger detector--the size of a fingerprint image--is prohibitively expensive for most applications.
This is the motivation for considering tapers even though a taper in turn disadvantageously adds to the weight, size and cost of the apparatus. At the time of writing of Bowker, however, the CCD cost advantage in provision of a taper in many cases was more than offset by the incremental cost of the taper--even without considering the weight and size penalty.
At that time, it was not possible to predict reliably whether eventual cost relief should be expected in the detector or in the taper, or in neither. Unfortunately at the present writing, more than a year and a half later, that situation has not changed.
Accordingly for most miniaturized applications the trade-off solutions taught in Bowker remain uneconomical. It is still anticipated that those solutions will in time become practical, as the price of conventional crystalline-silicon CCD arrays in this size range may fall--perhaps partially in response to competition for usage in apparatus according to the present invention--or an alternative optical detector, such as for instance a self-scanned diode ("SSD") array or thin-film (noncrystalline) photosensor arrays, may become available at significantly lower cost.
Meanwhile a practical package embodying the better-illuminated fiber-optic prism taught in Bowker has not appeared, heretofore.
Self-contained Print Verifiers
While many fingerprint analyzers are available in desktop or countertop modules, no prior art teaches a satisfactory fingerprint reading and analyzing apparatus that is self contained (which, for purposes of this document, is to be understood as meaning at least self contained except for power source). Such apparatus is a necessary first step toward real-time fingerprint verification systems operable within either hand weapons or other tightly constrained volumes such as mentioned below.
Extremely small, self-contained print verifiers present special challenges: extremely high optical, electronic and logical precision are required in a tiny but rugged system--at very low price. These challenges have not been adequately addressed in the art.
Data Isolation And Incompatibility
A special problem of such self-contained systems is how to make the greatest use of data. This issue arises because system operations include taking original data, both from authorized users and candidate users.
In either of these cases, information about the fingerprint that has been read by the apparatus may later be needed or desirable for other purposes. Such use was previously suggested in connection with anamorphism in the data.
First, where a home or business has many locks, it may be desirable to take authorized-user data just once--using just one of the locks--and then electronically copy the information into all the others. Second, law-enforcement agencies may have a particular use for such data.
This latter situation may arise for example when a facility has been entered forcibly and there is reason to believe that the intruder first attempted to operate the fingerprint-controlled lock. It may also arise when a person who has been an authorized user steals from the controlled facility, or commits some other crime--whether there or elsewhere. Other possibilities arise when an authorized user, for example a missing child, is not likely to have been otherwise fingerprinted.
Data export can be a problem in particular when a system operates using multilevel data, or using data in a special form such as sinusoidal or Fourier-transform data--as is the case for instance with Thebaud's, mentioned previously. Exporting such data may not be useful if the receiving application (such as law enforcement) that could use the underlying information operates on data in more-conventional formats.
Door Applications
While addressed broadly to many applications of a fingerprint reading device, Bowker gives particular attention to the configurations suited for use in guarding a weapon--particularly a small hand weapon. Mainly because of the cost considerations discussed above, hand-weapon applications appear to remain for the present just slightly beyond the range of economic development in commercial exploitation.
A market that is much more practical in view of the apparatus sizes that can be installed, and also taking into the number of now-unguarded units in use, is the protection of doors--and more particularly door handles. Although still small, a typical door handle and its associated lock have (at least potentially) several times greater volume for installation of security equipment than does a typical hand weapon.
Accordingly it is believed that the prior art has not adequately attended the opportunities for optical skin-pattern readers in direct association with doors, door handles and doorknobs.
Numerical Aperture Of Tapers
In Bowker it is taught that a taper used in the invention should be of relatively very high NA, certainly well over 0.5, to compensate for the intrinsic degradation in light-transmitting power associated with the image-reduction capabilities of a taper--and thereby allow transmission of enough optical energy to match the main part of the prism. The degradation is proportional to the square of the reduction; thus for example it is said that a two-times reducing taper should have NA.gtoreq.0.66, a three-times taper NA.gtoreq.1.05, and a four-times taper NA.gtoreq.1.4, in conjunction with a main-prism NA of 0.35.
This teaching, however, has since been recognized as partly in error. If a high-NA taper is employed to receive the optical image signal from a low-NA main prism (at least if this is done without special precautions), longitudinally diffusing stray light in the prism section can enter the taper. Such diffusing stray light arises, at the fingertip-contacting end face of the prism, from the excitation illumination which is reflected by that end face at steep angles relative to the fiber axes.
If the main prism is short, this adverse effect is aggravated--because the longitudinally diffusing stray light does not have adequate longitudinal diffusion distance in which to escape from the system, before reaching the taper. Since a high-NA taper by definition has high ducting capability, the stray light even though angled steeply--beyond the ducting range of the main prism--once into the taper is all carried to the sensor.
Such a result is undesirable because the diffusing stray light is uncorrelated with the signal in each fiber, and so badly fogs the image. The stray light can be quite bright, particularly in dark-field cases where it arises from the specularly reflected, unmonitored bright background.
Therefore new measures are needed to accommodate the poor optical signal-to-noise phenomena associated with feeding a high-NA taper from a short, low-NA main prism.
Applications
More generally the art has not heretofore provided an economical optical fingerprint reader module that is amenable to microminiaturization for access control in highly demanding field applications, particularly including common doors and door handles as well as personal weapons--and also encompassing access to use of portable computers and phones.
Time-and-attendance systems, database access systems, public phones, phone credit systems, vehicles, automatic tellers and facility-entry access devices, although not as critical as portable personal equipment or self-contained door-handle systems in terms of size, time, power, identification certainty, etc. would also be meaningfully enhanced by provision of a self-contained microminiaturized reader.
As now seen, the art has not yet provided solutions to important problems; and important aspects of the technology in the field of the invention are amenable to useful refinement.